In this thesis, we describe work we carried out to extend our knowledge on instrumental sources of MRI variability, in both anatomical and functional imaging. The anatomical study involved five adults scanned four times in the same platform MRI twice before and twice after a major update of a 3T Siemens scanner. The volumes of left and right hippocampus of each subject were measured with a manual segmentation. We then analyzed the test-retest reliability before and after updating the MRI system. In the functional study, five adults were scanned four times on the same MRI system twice before and twice after a major hardware update. Blood oxygenation level- dependent (BOLD) acquisitions were made at two different spatial resolutions (2x2x2mm and 4x4x4mm) to assess the reproducibility of BOLD measurements under conditions of high and low SNR. A final functional study using a gel phantom was used to study the stability of the machine for functional images and isolate the sources of noise arising from hardware. Two-dimensional gradient-echo EPI (Echo Planar Imaging) signals were analyzed to assess the percentage variability and Fourier analysis of residual error performed. Our results indicate that the differences in hardware from a major upgrade do not significantly affect the reliability of structural and functional studies spanning a major hardware upgrade. Daily quality assurance acquisitions on phantoms can be used to monitor the stability of the machine, which may aid in the early detection of hardware faults.